1. Field of the Invention
This invention relates to an exposure control device employed in an image sensing apparatus.
2. Description of the Related Art
The recent advancement of video apparatuses such as a video tape recorder (hereinafter referred to as a VTR for short) is conspicuous. As a result, the camera-integrated type VTRs or the like are generally equipped with automatic exposure adjusting devices.
FIG. 1 of the accompanying drawings shows a typical example of the conventional exposure control method heretofore employed for a camera-integrated type VTR. Referring to FIG. 1, a light flux coming through an optical system 1 forms an object's image on a image sensor 3 after the quantity of light is adjusted by an exposure adjustment device 2 (hereinafter referred to as an iris). A signal thus obtained from the image sensor 3 is supplied to a camera signal processing circuit 4. The circuit 4 processes the signal including gamma correction, etc. and separates the signal into a luminance signal and a chrominance signal. A camera encoder 5 is arranged to convert these outputs of the camera signal processing circuit 4 into a TV signal which is in conformity with, for example, the NTSC system. Meanwhile, an image luminance signal Y which is obtained from the camera signal processing circuit 4 without undergoing the gamma correction process is supplied to an integrator 6. The integrator 6 performs an integrating process. The integrated luminance signal is supplied to an arithmetic unit 7. The arithmetic unit 7 generates a difference signal indicating a difference between the luminance signal and a predetermined reference value 8. The difference signal is supplied as an exposure control signal to a driver 9. In accordance with the exposure control signal, the driver 9 and an actuator 10 control the aperture position of the iris 2 (a stop device for exposure adjustment) in such a way as to bring the output of the integrator 6 into a given relation to the predetermined reference value 8.
The conventional automatic exposure control device, however, has presented the following problems:
An exposure intended by the photographer does not always turn out to be apposite to an object to be photographed. For example, in cases where the object has a light source in the background under a so-called counter-light condition or where the object is solely illuminated in a dark place, the video camera performs light measurement by averaging the light of a wide area including the background besides the object. Therefore, the camera makes an under-exposure in the former case and an over-exposure in the latter case.
To solve this problem, some of the video cameras of the kind having automatic exposure control devices is provided with manual exposure control means for permitting the photographer to manually set the camera into a desired exposure effecting state by switching it from an automatic exposure control mode over to a manual exposure control mode in cases where the desired exposure is considered to be hardly possible with the automatic exposure control device. However, at the time of switch-over from an automatic exposure operation to a manual exposure operation of the conventional video camera of this kind, the iris is operated incontinuously from its state obtained before the switch-over and thus gives an unnatural picture.
Further, a second problem of the conventional automatic exposure control device lies in that: An aperture control signal is obtainable only for a linear process. In response to a change in the signal, the moving degree of the actuator changes to a degree in relation to the stopping-down degree of the iris and not in exact conformity to the signal change. The actuator moves to a less degree on the maximum aperture side and to a greater degree on the minimum aperture side.
Considering this in respect to a feedback system including the video signal, the feedback loop gain of the system increases accordingly as the iris position is stopped down from its full open state and decreases accordingly as the iris is opened from a stopped-down aperture position. In other words, the quantity of light varies to a less degree when the iris aperture changes within a range near the full open position of the iris. However, the luminosity on an image sensing plane varies to a greater degree accordingly as the iris position comes nearer to the minimum aperture in response to even a very small change in the aperture. This causes a change in the responsivity of the feedback loop of iris control. Therefore, the moving degree of the actuator tends to increase accordingly as the iris aperture is stopped down. This not only causes hunting on the side of stopped-down aperture values but also prevents smooth aperture control.
FIG. 2 shows another example of the conventional automatic exposure control device including manual exposure control means. The same component parts as those of FIG. 1 are indicated by the same reference numerals.
Referring to FIG. 2, a light flux which comes through an optical system 1 forms an object's image on an image sensor 3 after light quantity adjustment by an exposure adjustment device (or an iris) 2. A signal obtained by the image sensor 3 is supplied to a sample-and-hold circuit 11 to be sampled in a given cycle. The sampled signal is supplied to a camera signal processing circuit 4 to be subjected to various processes including gamma correction, etc.. At the circuit 4, the signal is separated into a luminance signal and a chrominance signal. These signals are supplied to a camera encoder 5 to be converted into a TV video signal which is in conformity to, for example, the NTSC system.
Meanwhile, an image luminance signal Y which is also output from the sample-and-hold circuit 11 is integrated by an integrator 6. The integrated signal is supplied to an arithmetic unit 7 to be compared with a reference value 8a predetermined by a reference level setting variable resistor 8. The arithmetic unit 7 then generates a difference signal according to a difference between the two. The difference signal is supplied as an exposure control signal to a driver 9 and an iris driving actuator 10. The aperture of the iris 2 is controlled in such a way as to have the output level of the integrator 6 in a give relation to the reference value which has been set by the reference level setting variable resistor 8. The iris 2 is thus controlled by the above-stated feedback loop.
A variable resistor 12 for manual exposure control is arranged to permit manual control over the exposing state of the iris 2. The set value of the above-stated reference level setting variable resistor 8 can be changed by operating the manual exposure control variable resistor 12. The aperture position of the iris 2 is thus manually adjustable to the set reference value.
However, the above-stated device has presented a third problem which is as follows: According to the arrangement of the device, the voltage of the reference value 8a of the reference level setting variable resistor 8 is simply varied while the position of the iris is always varying with the quantity of incident light. It is, therefore, difficult to set the camera in the optimum exposure state as desired.
Further, the iris control means for controlling the aperture in the manual exposure control mode is formed solely with the variable resistor which varies the reference value to be supplied to the feedback loop. It is, therefore, difficult to smoothly and finely adjust the aperture to the optimum value in the manual mode.
Meanwhile, the moving degree of the iris controlling actuator which is arranged to move in response to a change in the iris control signal varies in relation to the opening degree of the iris. The actuator, therefore, moves to a less degree on the side of larger opened aperture values and to a greater degree for stopped-down aperture values. FIG. 8 shows this relation of aperture values to the moving degree of the actuator.
In the case of the feedback system of the above-stated device including the video signal, the loop gain of the feedback loop increases accordingly as the iris is stopped down from its full open position and decreases accordingly as it is opened from the stopped-down position. In other words, the quantity of light changes to a less degree in response to a change in the iris aperture when the iris is in a position near to its maximum aperture. However, the light quantity changes to a greater degree to bring about a salient change in the luminosity of the image sensing plane, in response to even a slight degree of change in the aperture, accordingly as the the aperture is stopped down further. This causes a change in the responsivity of the feedback loop of iris control. The moving degree of the actuator thus tends to increase accordingly as the iris is stopped down to a greater extent. As a result, hunting takes place when the iris is within the range of stopped-down aperture values. Besides, the iris control cannot be smoothly accomplished.
When the iris is,manually controlled in particular, the actual stopping-down degree of iris aperture obtained in response to a certain degree of operation while the iris is in a smaller aperture position greatly differs from the degree obtained in response to the same degree of operation while the iris is within the range of larger aperture positions. This makes it difficult to obtain an optimum value by manual exposure control.
Further, the automatic iris control system of the conventional video camera of this kind has presented a fourth problem which is as follows: The circuit of the system has been arranged to have the same gain both for a change of the aperture of the iris (stop) from a maximum aperture to a smaller aperture according to the brightness of the object and for a change of the aperture from the smaller aperture toward the maximum aperture.
However, in accordance with the above-stated conventional method, the apparent loop gain of the automatic iris control system is higher when the brightness of the object changes, for example, from a dark state to a bright state, to cause the iris 2 to change from the maximum aperture position to a stopped-down (smaller) aperture position than when the the object's brightness changes from a bright state to a dark state to cause the iris 2 to change from a smaller position toward a maximum aperture position. The response speed of the operation of the system obtained in changing the iris 2 from a larger aperture to a smaller aperture differs from the speed obtained in changing it from a smaller aperture to a larger aperture.
Therefore, if the circuit gain is determined in such a way as to prevent the iris operation from hunting in changing a large aperture to a smaller aperture, the response speed becomes too slow and results in an unnatural automatic iris control action in changing the iris from a smaller aperture to a larger aperture.